In a domestic 22.9 KV-Y distribution system, an overhead earth wire support is used for lightning shielding and an overhead earth wire is installed in a bitt shape, which is vulnerable to natural disasters, on an upper power of an extra-high voltage power line, and an extra-high voltage neutral line, which is a return path of an unbalanced current or a fault current, is installed below the extra-high voltage power line.
The overhead earth wire and the extra-high voltage neutral line are connected to each electric pole to perform same functions of an electric parallel circuit of a circuit grounded at 50 ohm or less at an interval of 200 m. However, the overhead earth wire and the extra-high voltage neutral line may be redundantly installed at different positions of the electric pole together with a communication line facility, so the load applied to the electric pole may be increased. In addition, the overhead earth wire uses a galvanized steed stranded cable having a high impedance, thus the wire is frequently disconnected because the wire deteriorates from lightning, unbalanced currents and fault currents, so the construction must be performed by taking into consideration the type, thickness and conductivity of the wire. Further, when a low-voltage load current exceeds an allowable current of the extra-voltage neutral line in a low-voltage jointed area, in which the transformer is installed and the extra-high voltage neutral line and the low-voltage neutral line are used together, a secondary neutral line connecting point of the transformer may become disconnected.
According to an existing distribution line, as illustrated in FIG. 1, in case of equipment and a pole of an existing 22.9 KV-Y extra-high voltage distribution line, an overhead earth wire 40 is installed at a top of an electric pole in a bitt shape by using a galvanized steel stranded cable (FS22 mm2) and an overhead earth wire support, a power line 20 is installed at an upper part of the electric pole 10, a transformer 51 is installed in a low-voltage jointed area 50 of the power line 20, and a low and extra-high voltage neutral line 60 and a low-voltage line 80 are installed below the transformer 51 in the low-voltage jointed area 50.
In this case, the extra-high voltage neutral line 60 uses an aluminum cable steel reinforced (ACSR) wire having a thickness the same as or one-level lower than the thickness of the power line 20.
In addition, a bare hard-drawn copper wire WO is used for the extra-high voltage neutral line in places where wire corrosion is severe due to salt such as coastal areas.
In addition, when a low-voltage load current exceeds an allowable current of the extra-voltage neutral line in a low-voltage jointed area, in which the transformer is installed and the extra-high voltage neutral line and the low-voltage neutral line are used together, a secondary neutral line connecting point of the transformer may disconnect. Therefore, in urban areas, the low-voltage neutral line is added or a bare hard-drawn copper wire WO, which has a high allowable current, is used for the extra-high voltage neutral line to prevent the secondary neutral line connection point of the transformer from being disconnected.
Meanwhile, in a 22.9 KV-Y neutral line multiple ground system, the number of ground places and a ground resistance value may exert great influence upon the lightning shielding, so the typical overhead earth wire and the extra-high voltage neutral line are connected to each electric pole and are grounded at 50 ohm or less at an interval of every 200 m (3 to 4 span) and the electric pole, in which the transformer is installed, is grounded at 25 ohm or less.
However, in the existing distribution line, the overhead earth wire may be disconnected due to overheating and deterioration, and unbalanced return currents may flow during lightning and normal conditions.
In addition, the secondary neutral line connecting point of the transformer becomes disconnected in the low-voltage jointed area due to the use of the extra-high voltage neutral line and the low-voltage neutral line together and the disconnection due to the connection between different types of metals of a secondary lightning conductor (copper) and an extra-high voltage neutral line (aluminum) has frequently occurred.
In addition, when installing communication lines with the distribution line, the distribution line becomes complex because the communication line is installed at the position of the extra-high voltage neutral line so the aesthetic feature of the urban area may deteriorate and construction costs may be excessively consumed, and due to the increase in independent communication networks, the load applied to the electric pole by the communication line and the electric line increases thus increasing the number of broken electric poles.
Meanwhile, the applicant has filed the Korean Registered Patent No. 10-0725382, which discloses an electric pole having neutral line combined with overhead earth wire and distribution method using the same. In the patent, an electric pole, in which the overhead earth wire and the neutral wire can be used by one electric power line without a danger of increase in surge voltage from direct lightning strokes, is provided, such that the neutral line can be used as the overhead earth line so the construction cost can be reduced, the line is simplified so the weight of the support material can be reduced, and a lack of a ground clearance of the neutral line can be solved.
Meanwhile, in current power businesses, the needs for establishing its own information communication network by the power supply company due to distribution automation, smart grid, etc. are increasing, and due to these needs, each communication company is permitted to install the communication line along with the power line on the electric pole, allows the communication line to be separately installed with the distribution line, and the power supply company leases the installed communication line for its own information communication network.
However, to establish the communication line, a separate communication line is required to be separately installed from the overhead earth wire and the neutral line. Accordingly, the degree of fatigue of the distribution electric pole increases due to the communication equipment installed with the power line, which exceeds the allowable load of the distribution line, thus increasing cracks and crumbling of the electric poles, In addition, construction costs and aesthetic feature of the urban area may be degraded, and specifically, the distribution electric pole licensee is required to separately pay a lease cost of the distribution automation communication line to the communication company.
Further, the integrated overhead earth wire and neutral line is grounded at 50 ohm at a span of 200 m, so the number of ground areas is small, but work required for lowering the ground resistance value is difficult.
As related arts, there is Korean Registered Patent No. 10-0725382.